1. Field of the Invention
This invention relates to a honeycomb core-laminate composite structure useful, for example, as a light weight panel or structural member in aircraft, watercraft, and housing and commercial structural applications. More specifically, this invention relates to a composite structure comprising a honeycomb structure filled with a polyquinoxaline foam with a layer of a polyimide resin impregnated noncombustible fibrous material laminated on a surface of the core. This invention also relates to a process for producing the above-described composite structure and to articles employing therein the above-described composite structure.
2. Description of the Prior Art
Typical prior art composite structures, generally used as interior panels in aircraft, watercraft, etc., particularly where light-weight materials are desirable, comprise a decorative surface bonded to a laminate on a honeycomb core. Such a core material known in the prior art for a "sandwich" composite generally utilizes a honeycomb structure whose cells have hexagonal shape (hereinafter, for simplicity, "honeycomb" structure). The cells of the honeycomb structure can be unfilled or can be filled with materials such as a polurethane foam or a phenolic microballoon-filled epoxy potting compound. Layers of materials, for example, pre-imgregnated glass fibrous materials are then adhesively laminated to the core and a decorative surface, for example, produced by silk screening can then be placed thereon in order to produce, for example, a panel material which can be used in the interior of aircraft, watercraft, etc. In the prior art a transparent polyvinyl fluoride film has been bonded, using polymethylmethacrylate, to the decorative surface to provide protection. The material generally bonded to the honeycomb core structure employed in the prior art is a glass cloth material pre-impregnated with an epoxy resin and on bonding of this to the honeycomb core structre, the glass cloth pre-impregnated with the epoxy resin can be subjected to a texturizing operation to provide a surface texture thereto.
In the aircraft industry, in particular, fires constitute a series threat to life and result in considerable economic losses to airlines and the airline industry. In addition, in order to minimize hazards, the flammability as well as the smoking and noxious gas generation characteristics of polymeric materials used in aircraft interiors have been of particular interest and research has been conducted to develop techniques whereby materials can be rendered non-flammable and whereby smoking and noxious gas generation by materials can be eliminated or at least minimized.
In addition to the production of structural members and composite panels as described above in which the flammability and smoking gas generation characteristics have been reduced, investigations of components employed in such materials, particularly those having desirably reduced flammability and smoking gas generation characteristics, have been the subject of interest in the prior art. For example, the production of polymeric materials and coatings to which fire retardancy additives such as antimony oxide, various phosphorus compounds, and the like have been incorporated to render the materials non-flammable are well known in the art. In addition, materials which intumesce upon heating, thereby decreasing the rate of flammability or generating gaseous materials which retard the burning are also well known. Intumescent materials have also been found to be advantageous due to the increase in thermal insulation characteristics which arise upon intumescence.
More specifically, U.S. Pat. Nos. 3,535,130 and 3,663,464 disclose an intumescent material for a coating, for example, as a paint, useful in fire protection. The intumescent material is an aromatic nitroamino compound, such as p-nitroaniline, generally in a salt form, dissolved or dispersed in a vehicle, or alternatively pre-polymerized and dissolved or dispersed in a vehicle. The disclosure is that upon heating, the coating intumesces, chars, and provides a flame-resistant coating. Unfortunately, sufficient flame-resistance is not achieved using this type of intumescent coating. Further, large volumes of smoke are generated and there is the possibility of the formation of toxic gases such as nitroaniline compounds.
U.S. Pat. No. 3,811,997 discloses smoke and flame resistant structural laminated articles comprising a thin film or an aromatic polyamide or polyimide laminated over an impregnated fibrous sheet, which may be optionally laminated over a honeycomb structure. In this patent, the polyimide/aromatic polyamide film protective surface sheet is a replacement for the prior art use of a metal foil on a light-weight honeycomb structure. Although such is an improvement over the prior art use of a metal foil since the replacement of the metal foil by such a film reduces the weight of the structure, again the film of the aromatic polyamide or the polyimide does not provide sufficient fire resistance. In particular, in a high heat flux environment, for example, of about 11 .times. 10.sup.4 w/m.sup.2, only minute protection to the composite structure is provided by the thin film.
As can be seen from an examination of the above, of which the references cited are representative of developments in the art directed toward improving the non-flammability characteristics, smoke and noxious gas generation characteristics, and light-weight nature of materials, it is desirable to have light-weight, non-flammable and non-noxious gas generating materials which can be used as structural members or panels in the aircraft industry, the ship-building industry and the domestic and commercial construction industry, particularly the aircraft industry.